In recent times, ferrite and magnetite particles have been coated with metal to provide conductive materials with good electrical and magnetic properties. It is contemplated by this invention that these types of materials can be used in shielding applications.
The current invention reflects the discovery that, when ferrites are mixed with certain polymers, they provide easily extrudable compounds most suitable for wire and cable fabrication. Such compounds can be directly extruded over bare or insulated wire to form a wire and cable article that attenuates (filters) high frequency interferences.
This invention features wire and cable insulation that can be fabricated for attenuating lower and higher frequency interference signals than were heretofore possible. The extended frequency range encompasses both RFI and EMI frequency signals. The insulation layer of the invention provides both RFI and EMI frequency attenuation in a single layer, without the need for metal braiding.
Wire providing microwave/radar frequency attenuation is referred to in the wire and cable trade as "filter line." Some of these cables are referenced by U.S. Mil Spec. No. Mil-C-85485. The measurement of the attenuation (insertion loss) upon a given wire's performance relates to the effect filter line provides upon interference signals conducted down the wire.
Properly shielded filter line provides protection against radiated EMI. Noise currents and voltages are induced on the conductors of the cables when a radiated field causes interference. Filter line can attenuate such noise when it is shielded by metallic braid or other forms of conventional shield layering. The shielding effect can be measured by transfer impedance techniques.
The present invention seeks to fabricate wire and cable articles that provide protection against both aforementioned effects (i.e., attenuation of signals conducted down the wire, and radiated EMI) utilizing only a single layer of material.
The current invention contemplates a wire or cable construction employing a layer consisting of silver-coated magnetic particles such as ferrites and magnetites dispersed in a polymeric matrix, such as Viton, a fluorinated elastomeric polymer manufactured by DuPont. The magnetic particles are manufactured by various industries such as Steward Mfg. Co. of Tenn., and Fair-Rite Products Corp. of N.Y. The impedance characteristics of the magnetic particles vary depending upon the supplier, fabrication conditions, and composition. Metal coating such as silver is provided by Potters Industries, Inc., of Parsippany, N.J.
High frequency signals conducted down this wire are partially absorbed by the silver-coated particle shield layer. The electromagnetic waves penetrate through this shield layer up to the ferrite particles, and are then dissipated by lattice vibration or photon emission. Protection against radiated EMI is provided by the same shield layer via the percolating structure that consists of a large metallic surface area (silver coatings on each particle). The resulting noise created by the electromagnetic wave, therefore, is absorbed by the silver coating component of the ferrite particles. Thus, a filter line cable is provided which does not require an additional metal braid shield layer. The advantages of such a construction include a savings in cost and weight, and an improved flexibility compared to metal shielded wire utilizing tapes, braids, foils, etc. Weight saving is particularly important in view of the stringent requirements for present day, light-weight space and aeronautical wire and cable.